New synthesis strategies allowing to obtain materials of well-defined porosity in a very wide range, from microporous materials to macroporous materials to hierarchical porosity materials, i.e. having pores of several sizes, have known a very large development within the scientific community since the mid-90s (G. J. de A. A. Soler-Illia, C. Sanchez, B. Lebeau, J. Patarin, Chem. Rev., 2002, 102, 4093). Materials whose pore size is controlled are obtained. In particular, the development of synthesis methods referred to as “soft chemistry” has led to the elaboration of mesostructured materials at low temperature through the co-existence, in aqueous solution or in polar solvents, of inorganic precursors with structuring agents, generally molecular or supramolecular surfactants, ionic or neutral. Control of electrostatic interactions or through hydrogen bonds between the inorganic precursors and the structuring agent jointly linked with hydrolysis/condensation reactions of the inorganic precursor leads to a cooperative assembly of the organic and inorganic phases generating micelle aggregates of surfactants of uniform and controlled size within an inorganic matrix. This cooperative self-assembly phenomenon governed, among other things, by the structuring agent concentration, can be induced by progressive evaporation of a solution of reactants whose structuring agent concentration is in most cases lower than the critical micelle concentration, which leads to either the formation of mesostructured films in the case of a deposition on substrate (dip-coating technique) or to the formation of a mesostructured powder after atomization (aerosol technique) or draining of the solution. By way of example, U.S. Pat. No. 6,387,453 discloses the formation of mesostructured organic-inorganic hybrid films by means of the dip-coating technique, and these authors have furthermore used the aerosol technique to elaborate mesostructured purely silicic materials (C. J. Brinker, Y. Lu, A. Sellinger, H. Fan, Adv. Mat., 1999, 11, 7). Clearance of the porosity is then obtained by surfactant elimination, which is conventionally carried out by means of chemical extraction processes or by thermal treatment. Depending on the nature of the inorganic precursors and of the structuring agent used, and on the operating conditions applied, several families of mesostructured materials have been developed. For example, the M41S family initially developed by Mobil (J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T.-W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins, J. L. Schlenker, J. Am. Chem. Soc., 1992, 114, 27, 10834), consisting of mesoporous materials obtained using ionic surfactants such as quaternary ammonium salts, having a generally hexagonal, cubic or lamellar structure, pores of uniform diameter ranging from 1.5 to 10 nm and amorphous walls of thickness of the order of 1 to 2 nm, has been widely studied. Later, structuring agents of a different chemical nature have been used as amphiphilic macromolecules of block copolymer type, the latter leading to mesostructured materials having a generally hexagonal, cubic or lamellar structure, pores of uniform diameter ranging from 4 to 50 nm and amorphous walls of thickness ranging from 3 to 7 nm.
In addition to the synthesis techniques using dip-coating or formation of a powder (aerosol/draining) described above, which use the phenomenon of progressive concentration of the inorganic precursors and of the structuring agent within the solution where they are present, the mesostructured materials can be obtained by direct precipitation within an aqueous solution or in solvents of marked polarity by using the value of the critical micelle concentration of the structuring agent. Generally, synthesis of these materials obtained by precipitation requires a ripening stage in an autoclave and all the reactants are not integrated in the products in stoichiometric proportion since they can be found in the supernatent. Depending on the structure and on the organization degree required for the final mesostructured material, these syntheses can take place in an acid medium (pH≦1) (WO-99/37,705) or in a neutral medium (WO-96/39,357), the nature of the structuring agent used also playing an essential part. The elementary particles thus obtained have no regular shape and they are generally characterized by a size well above 500 nm.
The discovery of these materials of uniform and organized porosity has opened up new prospects as regards the elaboration of innovative solids for potential applications in such varied spheres as catalysis, chemical molecules separation, as well as optics, electronics and biochemistry. In particular, the study of the introduction of metallic nanoparticles in essentially silicic mesostructured oxide matrices has led to a large number of publications and patents. In fact, using such a host network during the synthesis of metallic nanoparticles has contributed to the following scientific advances: better control of the size and of the morphology of the metallic nanoparticles leading, in the sphere of catalysis for example, to new activities and selectivities according to the reactions studied, and better dispersion of the metallic nanoparticles within the support by means of a promoted diffusion of the metallic precursors due to the organization of the host network porosity. One of the conventional methods allowing incorporation of the metallic nanoparticles in a mesostructured silicic network consists in synthesizing, in a first stage, the host network according to the synthesis methods described above, then, in a second stage, in forming within the porosity thus created metallic nanoparticles according to the following non-exhaustive methods: impregnation of precursor inorganic salts, exchanges of metallic ions with ions present at the surface of the host network, grafting of organometallic complexes, metallic crystallites (also referred to as clusters) or preformed nanoparticles stabilized by organic ligands, etc. This method also allows elaboration of mesostructured essentially silicic solids having, within their pores, nanoparticles of gold, noble metals, iron oxide, silver oxide, etc. (A. Fukuoka, H. Araki, Y. Sakamoto, S. Inagaki, Y. Fukushima, M. Ichikawa, Inorganica Chimica Acta, 2003, 350, 371; S. Behrens, G. Spittel, Dalton Trans., 2005, 868; K.-J. Chao, M.-H. Cheng, Y.-F. Ho, P.-H. Liu, Catalysis Today, 2004, 97, 49; M. Fröba, R. Köhn, G. Bouffaud, Chem. Mater., 1999, 11, 2858). Another option consists in introducing the desired nanoparticles directly upon elaboration of the mesostructured host network. It is thus possible to introduce in the micelles formed by the structuring agent, during synthesis, metallic nanoparticle precursors by using their hydrophobic or electrostatic properties (G. Lü, D. Ji, G. Qian, Y. Qi, X. Wang, J. Suo, Applied Catalysis A: General, 2005, 280, 175; Ö. Dag, O. Samarskaya, N. Coombs, G. A. Ozin, J. Mater. Chem., 2003, 13, 328). All these methods however lead to partial or even total obstruction of the porosity of the mesostructured matrix, which eventually does not allow to take advantage of both the textural and/or structural properties of the mesostructuration and of the metallic nanoparticles.